1. Field
The field relates to a current mode DC-DC converter, more particularly, it relates to a current mode DC-DC converter with excellent high-speed responsiveness with respect to fluctuations in the output voltage.
2. Description of Related Art
A DC-DC converter disclosed in Japanese Patent Publication No. 2005-86992 is constituted of a switching power source unit 110 in which a switching element capable of being turned ON/OFF at a high frequency is turned ON/OFF to convert an input voltage Vin of a direct current into an output voltage Vout of a direct current, and a control unit 112 that controls the ON/OFF operation of the switching element provided in the switching power source unit 110.
The control unit 112 comprises an error amplifier 114, a switching control circuit 116, a bias current selection circuit 122 and a resistance voltage dividing circuit 144. Further, the error amplifier 114 comprises a differential amplifier 138 consisting of operational amplifiers, a constant current source circuit 140 that supplies a bias current to each unit inside the differential amplifier 138, and an output circuit connected between an output terminal of the differential amplifier 138 and a ground potential.
The differential amplifier 138 comprises a differential input unit that generates a differential voltage of an input voltage, and a voltage amplifying unit and the like that amplifies a differential voltage supplied from the differential input unit. These units are adapted to operate under a bias current I supplied from the constant current source circuit 140. Thus, as the bias current I increases, the response sensitivity or response speed of the differential amplifier 138 also increases.
The constant current source circuit 140 comprises at least two independent constant current source circuits 140A and 140B connected in parallel. A first constant current source 140A steadily supplies a constant bias current IA to each unit provided in the differential amplifier 138. The second constant current source 140B is controlled to ON/OFF by the bias current selection circuit 122A to be described later, so that the second constant current source 140B is kept in an OFF state when the error amplifier 114 has a first response characteristic and is switched to an ON state when the error amplifier 114 has a second response characteristic.
The bias current selection circuit 122 compares the feedback signal KVout obtained in the switching power source unit 110 and outputted from the resistance voltage dividing circuit 144 with an upper limit monitoring value AM, and selects IA as the bias current to be supplied from the constant current source circuit 140 to each unit provided in the differential amplifier 138, when KVout≦AM (the second constant current source 140B is kept in an OFF state), and selects the (IA+IB) when KVout>AM (the second constant current source 14B is placed in an ON state).
When the direct current output voltage Vout is out of the range of the upper limit monitoring value AM, the response characteristic of the error amplifier 114 is switched to the one with higher responsiveness, whereby the switching control operation in the control unit 112 is continued without stopping the switching control operation of the control system. As a result, the system can respond quickly and suitably even in the case of abrupt and significant fluctuations in the input voltage in normal operating conditions, without causing any interruptions in the operation of the control system.
Additionally, another similar technology is disclosed in the Japanese Patent Publication No. 2005-287165.
In the technology disclosed in the Japanese Patent Publication No. 2005-86992, the response characteristic of the error amplifier 114 is switched from the bias current IA to (IA+IB) when the direct current output voltage Vout is out of the range of the upper limit monitoring value AM, thereby switching to the characteristic with higher responsiveness to carry out the switching control operation. However, even when switching to the characteristic with a higher responsiveness, the responsiveness gradient is changed up to several times only, which creates a problem that the system may not be capable of adequately responding to an abrupt change in the output voltage.